1. Technical Field
The present invention relates to curve speed control systems, and more particularly to a speed control system configured to facilitate proper operator management of the curve by accelerating or decelerating the vehicle automatically.
2. Background Art
Particularly with respect to automobiles, operator mismanagement of a curve is one of the most prevalent factors contributing to accidents. In these accidents, it is appreciated that excessive lateral forces result when the rate of speed of the vehicle exceeds the rate of speed at which the vehicle could safely or comfortably manage the curve. It is further appreciated that when approaching a curve at excessive speeds the operator typically begins a precarious cycle of excessive braking and steering to resolve the cornering difficulties.
Historically, licensed civil engineers have designed thoroughfare curves by selecting one of a plurality of curve templates, i.e. circular, spiral, and more likely a combination of both, that best fits existing terrain and avoids obstructions for which removal is deemed too costly. The beginning of the curve is typically set at a given station and further indicia, such as the curve beginning and ending radius, is also typically noted on the plans. Computer aided design techniques and software provide cross-sections at typical station offsets, wherein elevation points and bank angles for the curve are represented. These plans are precisely staked and constructed in the field by survey and construction crews. Finally, the thoroughfare speed limit is determined, such that a typical driver and vehicle combination producing a minimal normal force, and lateral coefficient of friction with the surface, is capable of withstanding the lateral acceleration caused by centrifugal force acting upon the vehicle.
More particularly, centrifugal force, Fc(=mac=mv2/R), acts upon the vehicle during curve management to effect a laterally outward acceleration. To maintain the curve, i.e. constant radius, of the vehicle path, this force must be directly proportional to an equal and opposite centripetal force. With respect to automobile travel, the force of friction between the tires and the road surface provides centripetal force. To accommodate for conditions where friction is insufficient (e.g., on wet roads, ice, oil, etc.), the curve is preferably banked at an angle, θ, so that at least a portion of the centripetal force is provided instead by a normal force, FN (=mg). Equating Fc and FN on a normal road condition where friction is assumed to be 1, the maximum allowable velocity is related to gravity by: v2=g R tan θ, where g is the acceleration due to gravity, and R is the radius of curvature.
Thus, when a vehicle is speeding and friction is insufficient, it is often difficult for an operator to safely maneuver around a curve. To address curve mismanagement, systems have been developed to either identify an approaching curve or modify some aspect of the vehicle performance during or approaching the curve. Some of these systems present mechanisms and control logic for selecting and achieving an optimal transmission gear during curve management, and defining and estimating an approaching curve. Other systems determine stable running speeds for detected nodes and decelerate or accelerate the vehicle, so as to achieve the stable speed at a given point.
These conventional systems, however, are rigid one-size-fits-all models that do not enable modifications due to operator preference or vehicle characteristics. These systems also do not provide means for properly addressing special conditions that may modify an allowable curve speed profile where desired. Of yet further concern, these systems do not accommodate an instantaneous change in curvature radius that may occur at a circular curve termination point, nor provide feedback to enable the optimization of performance, and therefore may result in errors or rapid acceleration when exiting a curve.